This invention relates to the production of a woven material, usually in the form of a layered assembly, as a reinforcement for structural components, and is particularly concerned with the provision of a woven material of the above type, which when impregnated with a suitable thermosetting resin to form a layered article, is resistant to catastrophic failure or damage from shock or hard local impact.
Most shaped structural type composite parts are made from multiple plies or layers of woven cloth or unidirectional type fiber (graphite, boron, Kevlar, fiberglass or mixtures of such materials), impregnated with "B" stage thermosetting resin, such as an epoxy resin. In fabricating such composites, layers of material are laid up one layer at a time in a prescribed fiber ply orientation pattern to form a laminate having a total thickness and fiber direction to carry the structural load required for the completed article. This layered assembly is then impregnated with a "B" stage resin if the individual layers were not previously impregnated with such resin, and the assembly cured under heat and pressure by conventional processes, dependent on the resin system in use. Parts or panels fabricated in this manner, particularly when employing woven graphite of high modulus, impregnated with a thermosetting resin such as an epoxy, have outstanding mechanical properties such as tensile and compression strength and stiffness. These fiber reinforced resin or plastic articles have numerous applications, particularly as structural components in airplanes, ships and automobiles.
However, cured laminate parts or panels particularly formed of high modular fibers such as graphite, have certain brittle characteristics and are easily damaged from shock or local impact as from a hard object, forming cracks or fissures resulting from the high modulus of these fibers, that is, their weak ability to stretch elastically. This damage can spread or propagate across a panel and cause catastrophic failure at greatly reduced loading compared to an undamaged panel. Further, holes drilled in such panels for attachment, e.g. of rivets, bolts, hinges, and the like, also cause similar loss in panel strength by creating areas of reduced shock and impact resistance.
The art of weaving highly oriented fibers such as fiberglass, graphite, boron or Kevlar, in the warp direction and employing a small number of tie yarns such as fiberglass, Kevlar or graphite in the fill direction is known in the art, as illustrated by U.S. Pat. No. 3,997,697. This patent discloses a complex woven unidirectional warp reinforcement, e.g. of boron thread, having in the fill direction loosely woven continuous fine diameter organic fiber threads that hold the warp reinforcement in a parallel cohesive unit.
French Pat. 2,034,787 discloses a woven material with major reinforcements in the unidirectional warp direction of the cloth, composed of a uniform mixture of high modulus graphite and low modulus glass fibers, held together in the fill direction in a loose manner with a continuous fine glass fiber thread which holds the unidirectional reinforcement in a parallel and cohesive unit, to provide a low cost material as compared to an all graphite woven material.
U.S. Pat. No. 2,836,529 discloses a woven unidirectional fabric consisting of alternate fibers of glass and metal with transverse metal fibers included in the woven cloth.
It is an object of the present invention to provide a woven fiber material as reinforcement which can be impregnated with resin and employed as a structural component having increased resistance to damage. Another object of the invention is to provide a woven material layered assembly as reinforcement for production of resin impregnated articles which are resistant to catastrophic damage from shock or impact from a hard object.
Still another object of the invention is to provide an improved woven material, as in the form of a layered assembly of the type described above, which when impregnated with a suitable thermosetting resin, forms an article having superior retained strength when in a minor damaged condition, and is then further loaded by fatigue or static stress, and hence is tolerant to further damage.
A still further object is the provision of a woven material for forming a layered article which when impregnated with a suitable resin forms structural parts or components which permit the drilling of holes for the placement of rivets, hinges and the like, without reducing the retained mechanical properties of the layered assembly.